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M70 speaker distortion

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johndon
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johndon posted on Sun, Aug 20 2017 7:13 AM

I have a small scratchy distortion in one speaker. It's more obvious when playing piano music. I know it's the speaker because when I swap the leads at the amplifier the scratchy sound is still there in the same speaker. Can anyone give me a clue where to start ? It sounds like it's in the mid range speaker. Should I start with the speaker or the crossover board?

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AudioOrigin
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probably the spider is not supporting the cone properly, put the speaker upside down, does that fix it ? if so take out the offending driver mount upside down and it will be fine, it could be other things but this is likely if it has moved to much then this trick will not work and you need to get the driver repaired,


if you gently and evenly push the bass or mid driver (the one that looks like a small bass) it should be smooth if it catches then its out of alignment
the surround ageing a bit is also a possibility you should be able to see if that has happened, and repair if needed 
johndon
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Thanks for the tip!

 I took the speaker off the trumpet stand and turned it upside down, plugged the lead back in and bingo!  --- no distortion. I will now follow your suggestion with the mid range speaker and I'll post another reply to advise the final result.

Rich
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Rich replied on Tue, Aug 22 2017 1:14 AM
No! Is it the smaller cone, or the larger dome?

The smaller cone is the phase link, and the larger dome is the midrange. If it's the phase link, the magnet is beginning to slip. You'll need to epoxy the magnet. Well documented problem.


johndon
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Hi Rich

I might let sleeping dogs lie!!  Since I inverted the speaker box and then turned  it back the right way and reconnected everything, the distortion has disappeared. At some stage however, I plan to remove the front grill cloth from each speaker and carefully examine the speakers individually. After all, they are more than 40 years old.

chartz
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chartz replied on Wed, Aug 23 2017 8:55 AM

johndon:
After all, they are more than 40 years old.

And still going strong! Tim Jarman published a new review on them in the January 2017 issue of HFN&RR. You can order back issues if you want.

 

Jacques

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